VITAMINS 351 



phospholipides best known from brain tissue and as phytic acid, the 

 hexaphosphoric acid ester wliich occurs as the calcium-magnesium salt 

 j:)hytin in plants. Inositol is plentiful in heart muscle and in skeletal 

 muscle of sharks, where it may serve as an energy reserve similar to 

 glycogen in other species. It has been reported that inositol is a 

 prosthetic group of a-amylase, important in carbohydrate metabolism. 

 Roles have also been suggested for it in the metabolism of phenolic 

 compounds and ketones. 



Inositol is apparently required by chicks, turkeys, pigs, and a variety 

 of other animals. Daily allowances have not been established. De- 

 ficiency leads to loss of hair and under certain dietary conditions to 

 the accumulation of lipides in the liver. Meat, bran, yeast, and grains 

 are among the foods high in this factor. 



VITAMIN A 



This vitamin is a member of the group of food factors characterized 

 by their solubilities in fat solvents. The other important members 

 are vitamins D, E, and K. Certain fatty acids must also be supplied 

 in the diets of various animals and are discussed on pages 460 to 462. 



Chemistry 



Vitamin A is sometimes designated vitamin A^ to distinguish it from 

 two less common forms, Ao and A3, isolated from fresh-water fish and 

 lard, respectively. All are isoprenoids in that they can be represented 

 as consisting of isoprene units. Vitamin A^ is the most abundant, 

 occurring in vertebrates except in most fresh-water fish and the gill- 

 breathing forms of amphibians. Its structure is as follows: 



-CH3 ™^ 9^=* 



^— CH=CH— C=CH— CH=CH— C=CH— CHoOH 



CH3 CH3 



vitamin Ai 



Vitamin Ao differs in having another double bond in the upper left 

 position of the ring structure written above. A3 has been isolated only 

 recently and its structure is not completely established. With several 

 double bonds along the chain, a series of isomers of the cis-trans type 

 is possible. Two of these are known to be important. Trr/n^-vitamin A^ 

 has the trans configuration at all its double bonds, and the other type 



